google-cloud-nio: retry on 502 errors, and increase max depth when do…

…ing channel reopens (googleapis#3557)

We've frequently encountered transient 502 errors in the wild when using
google-cloud-nio (see broadinstitute/gatk#4888),
implying that this error should be added to the list of retryable errors.

I also increased the maximum depth when inspecting nested exceptions looking for
reopenable errors from 10 to 20, as we've seen chains of exceptions that come very
close to the current limit of 10.

google-cloud-clients/google-cloud-contrib/google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/CloudStorageRetryHandler.java

@@ -144,7 +144,7 @@ void sleepForAttempt(int attempt) {
    * @return true if exs is a retryable error, otherwise false
    */
   private static boolean isRetryable(final StorageException exs) {
-    return exs.isRetryable() || exs.getCode() == 500 || exs.getCode() == 503;
+    return exs.isRetryable() || exs.getCode() == 500 || exs.getCode() == 502 || exs.getCode() == 503;
   }
 
   /**
@@ -154,7 +154,7 @@ private static boolean isRetryable(final StorageException exs) {
   private static boolean isReopenable(final StorageException exs) {
     Throwable throwable = exs;
     // ensures finite iteration
-    int maxDepth = 10;
+    int maxDepth = 20;
     while (throwable != null && maxDepth-- > 0) {
       if ((throwable.getMessage() != null
           && throwable.getMessage().contains("Connection closed prematurely"))